The present invention relates to the design of optical filters, particularly multi-element spectral filters intended for use in wide field of view (WFOV) optical systems.
The purpose of an optical filter is to transmit radiation within some spectral band ("in-band") while rejecting radiation outside the band ("out-of-band"). To achieve the high degree of spectral discrimination needed by some optical systems, it is often necessary to construct a multi-element spectral filter having both interferring layers and absorbing materials. Typically, these filters have been constructed of planar elements and coatings combined in a sandwich arrangement. This configuration is only suited for cases where the range of incident angles of incident radiation on the filter is small.
In optical systems using spectral filters, it is often desirable that the transmittance of the filter be substantially uniform with respect to the angle of incidence of radiation on the filter (the "field angle.") However, in a WFOV system using a planar multi-element spectral filter, off-axis rays entering the filter traverse longer paths through the absorbing elements than do on-axis rays, so that the filter's transmittance varies as a function of the field angle. In addition, since most interference layer coatings are angularly sensitive, and since the angle of incidence at all surfaces equals the field angle for an all-planar configuration, the off-axis transmittance is further degraded.
In prior WFOV systems (for example, see U.S. Pat. No. 3,278,752) using planar multi-element spectral filters, a negative lens has been placed in front of the filter so that off-axis rays will pass through the filter with reduced angles of incidence. Although this approach provides a uniform angular response, it has the primary disadvantage that the collecting aperture is generally reduced in size, which results in a significant reduction in on-axis sensitivity.
It is, accordingly, a primary object of the present invention to provide an improved multi-element spectral filter for use in WFOV optical detection systems, which filter has substantially uniform angular response, increased sensitivity, has mild surface curvatures, is relatively small in size, and which is easily fabricated.